In a high frequency induction coupled plasma (or a hot plasma), the temperature of the center of the plasma flame is considerably high, usually higher than 5000.degree. C. Accordingly, the material of a torch forming a plasma flame must have high heat resistance and a low coefficient of thermal expansion. Thus, in general, such torches are made of quartz glass.
A conventional plasma torch is shown in FIG. 1. The plasma torch 1, is a multi-pipe torch which is formed as follows: Three cylindrical pipes 2, 3 and 4 are coaxially arranged; that is, the pipe 2 is inserted into the pipe 3, which is inserted into the pipe 4. The pipe 3 is welded to the pipe 2 at one end, and similarly the pipe 4 is welded to the pipe 3 at one end. Branch pipes 6 and 7 are extended from the pipes 3 and 4, respectively. A plasma gas and a cooling gas are introduced through the branch pipes 6 and 7, respectively. Accordingly, the inner wall of the torch 1, which contacts the plasma flame, is protected by a large quantity of cooling gas such as Ar or O.sub.2 gas. The upper portion of the plasma torch is inserted into an annular high frequency induction coil 5.
The plasma torch 1 thus constructed is manufactured by welding the cylindrical pipes 2, 3 and 4 as described above; that is, the manufacture of the plasma torch 1 requires a manual processing step. In addition, since the cylindrical pipes 2, 3 and 4 are comprised of quartz glass, the manual processing step must be carried out with considerable skill under high temperature. Accordingly, it is difficult to manufacture a plasma torch 1 with a high degree of accuracy, and it is especially difficult to manufacture a plasma torch in which the circumferential clearances between the cylindrical pipes 2, 3 and 4 are uniform. Therefore, in the conventional plasma torch 1, the plasma flame may be deflected. If deflection takes place, the pipe contacting the plasma flame is deformed or evaporated.
As is apparent from the above description, the conventional plasma torch does not generally possess sufficient accuracy, particularly with respect to the accuracy of the circumferential clearances between the pipes. This can cause various difficulties.